Thin-film electroluminescent device

Abstract: The thin-film electroluminescent device of the present invention comprises a thin film of a polyoxadiazole derivative represented by the formula: wherein each of X₀ and Y₀ independently represents a divalent organic group which may comprise a silicon atom, and n is an integer of 2 or greater, formed as an electroluminescent layer or a charge injecting/transporting layer between electrodes, at least one thereof being transparent.

Description: The present invention relates to an electroluminescent device or display device comprising an organic electroluminescent material for use in data display, optical data processing and other fields. In data display, optical data processing and other fields, recent attention has been drawn to organic electroluminescent materials. Electroluminescent materials emit light having a wavelength and an intensity characteristic of the material when it is sandwiched between electrodes and a voltage is applied thereto. It is believed that electrons and holes are injected from the electrodes into the organic electroluminescent material and moved in the organic electroluminescent material by the applied voltage, followed by recombination of the holes and electrons. The emitted light has a spectrum nearly identical with a fluorescence spectrum intrinsic for the electroluminescent material. Organic and inorganic electroluminescent materials are known. The organic electroluminescent materials include blue electroluminescence-inducing anthracene known from old. The inorganic electroluminescent materials include well known semiconductors. LED (light emitting diode) inducing red or yellow electroluminescence is now drawing attention as a highly bright and stable electroluminescent material. LEDs inducing red, orange and green electroluminescences are combined in practical use for a three-color display for streets. Appl. Phys. Lett., Vol. 51, No. 12 (1987), pp 913-915 describes a two-layer electroluminescent device (EL device) prepared with the use of organic electroluminescent materials. This two-layer electroluminescent device is prepared by successively forming on an electrode of ITO (indium tin oxide) a hole injecting layer, an electroluminescent layer capable of transporting electrons and an electron injecting electrode of an MgAg alloy by vapor deposition. When a voltage of several tens of volts is applied to this electroluminescent device, electrons and holes are injected into the electroluminescent layer to emit light. With this two-layer electroluminescent device, the color of the emitted light can be changed by choosing the type of the electroluminescent material. For example, a low molecular compound of an aluminum quinolinol complex (Alq3) used as the electroluminescent material provides a green electroluminescence. However, this two-layer electroluminescent device has a drawback in that the above low molecular compound deposited as an electroluminescent layer crystallizes causing detachment of the organic layer (electroluminescent layer) from the electrode, which thereby no longer emits light. There is another drawback in that the two-layer electroluminescent device generates heat accompanied with the emission of light which markedly increases the temperature of the device, so that the device is deteriorated. Saito et al. have proposed a three-layer organic electroluminescent (EL) device for improving electroluminescene efficiency (Jpn. J. Appl. Phys., Vol. 27, No. 2, 1988, pp. L269-L271 and Jpn. J. Appl. Phys., Vol. 27, No. 4, 1988, pp. L713-L715). This three-layer organic electroluminescent device is prepared by successively forming on an ITO electrode (positive electrode) a hole injecting layer, an electroluminescent layer, an electron injecting layer and an MgAg electrode (negative electrode). In this multi-layered structure, the hole injecting layer, the electroluminescent layer and the electron injecting layer are composed of organic layers. In this EL device, the injection efficiency of holes from the positive electrode into the electroluminescent layer is improved by the hole injecting layer, and the injection efficiency of electrons from the negative electrode into the electroluminescent layer is improved by the electron injecting layer. Consequently, the threshold voltage required for electroluminescence of the three-layer organic electroluminescent device is only several volts. Blue electroluminescence, which has been difficult to attain with inorganic materials, can be attained with this three-layer organic electroluminescent device at a threshold voltage comparable to that of the above two-layer organic electroluminescent device. However, this three-layer organic electroluminescent device uses organic low molecular weight compounds, such as oxadiazoles having a single oxadiazole ring, as materials capable of hole injection or electron transport. Most of these low molecular organic compounds have melting points as low as up to 300 °C. Therefore, disadvantageously, the three-layer organic electroluminescent device prepared from the above low molecular organic compounds has poor heat resistance and thereby suffers from thermal degradation of its properties, or recrystallization occurs in the device to cause degradation of its properties. It has been proposed to form at least one of the organic layers, which are the electron injecting (transporting) layer, the electroluminescent layer and the hole injecting (transporting) layer, from a polymeric thin film to avoid the above thermal degradation and recrystallization. For example, in JP-A-4 002 096, a process for producing a polymeric thin film EL device is proposed in which a polymeric thin film comprising an electroluminescent low molecular weight material or a low molecular weight material capable of hole injection and electron transport is formed by a wet process, such as spin coating or dip coating. Several tens of volts must be applied to the thus obtained device for providing an effective electroluminescent brightness. Even if this high voltage is applied, the electroluminescent brightness is only up to 200 cd/m 2 . In the formation of the above polymeric thin film by spin coating, there is a drawback in that pin holes are likely to form in the preparation of polymeric thin film. Pin holes cause the device to break during the driving of the device. Further, the formation of the polymeric thin film by the wet process is likely to introduce impurities into the device. The impurities accelerate the degradation of the device. Although the formation of a polymer layer by a wet process is easy, the process is not suitable for making the device. When the electroluminescent device is produced by forming an organic layer (upper layer) on an organic layer (under layer) according to the wet process, it is necessary to select a solvent which does not dissolve or leach the organic under layer in the preparation of a coating fluid for forming the upper organic layer. In the formation of an organic layer (upper layer) on an organic layer (under layer) according to the wet process, the materials usable for forming the under layer and the upper layer and the solvents for dissolving the materials are limited. Consequently, there is a problem that the types of polymeric materials capable of forming the organic layer of the polymeric thin-film electroluminescent device and the low molecular materials which can be contained in the polymeric materials are extremely limited. On the other hand, in EP-A-449125 (JP-A-3274693), it is proposed to use, as an electroluminescent layer or a charge injecting/transporting layer of an electroluminescent device, a thin film of a polyimide of formula: wherein X and Y are as defined below, and n is an integer indicating the degree of polymerization, the polyimide being prepared through a polyamic acid (polyimide precursor) by reacting an acid dianhydride of formula: wherein X represents an organic group including an aromatic group, with a diamino compound of formula: H 2 N - Y - NH 2 wherein Y represents an organic group having an EL function (at least one of charge injecting, charge transporting and electroluminescent functions) by a vapor deposition polymerization process. In the polyimide of formula (IX) the nitrogen atom forming sp 3 hybrid orbital and the carbon atom of the carbonyl group constitute a σ bond, which forms part of the principal chain of the polyimide. This σ bond restricts the π electron conjugation chain, so that excellent electron conductivity, thus high electroluminescence, cannot be expected from the electroluminescent device having a thin film of the polyimide as an electroluminescent layer or a charge injecting/transporting layer. Further, the polyimide of formula (IX) has a carbonyl group with a large dipole moment at the site of the imide bond. This carbonyl group is known to function as a trap of carriers (electrons and/or holes), thereby lowering the mobility of carriers. High electron conductivity cannot be expected from the polymeric thin film having a low carrier mobility. The present invention seeks to provide a thin-film electroluminescent device in which a polymeric thin film having not only excellent electroluminescence and/or capability of charge injection/transport but also excellent durability, such as heat resistance, is interposed between electrodes. The present invention provides, in a first aspect, a thin-film electroluminescent device comprising an electroluminescent layer interposed between electrodes, at least one thereof being transparent, wherein the electroluminescent layer comprises a polyoxadiazole derivative, formed by the vapor deposition polymerization process, represented by the formula: wherein each of X 0 and Y 0 independently represents a divalent organic group which may comprise a silicon atom, and n is an integer of 2 or greater. The present invention also provides, in a second aspect, a thin-film electroluminescent device comprising an electroluminescent layer interposed between electrodes, at least one thereof being transparent, and a charge injecting/transporting layer interposed between at least one of the electrodes and the electroluminescent layer, wherein at least one of the electroluminescent layer and the charge injecting/transporting layer comprises polyoxadiazole derivatives, formed by the vapor deposition polymerization process, represented by formula (I). In the drawings, Fig. 1 is a sectional view schematically illustrating the structure of a thin-film electroluminescent device of the first aspect of the present invention; Fig. 2 is a sectional view schematically illustrating a first form of a thin-film electroluminescent device of the second aspect of the present invention; Fig. 3 is a sectional view schematically illustrating a second form of a thin-film electroluminescent device of the second aspect of the present invention; Fig. 4 is a sectional view schematically illustrating a third form of a thin-film electroluminescent device of the second aspect of the present invention; Fig. 5 is a partial sectional view schematically illustrating the formation of the charge injecting/transporting layer comprising a silicon-atom containing polymeric compound and a polycondensed polymeric compound containing no silicon atom; and Fig. 6 shows a view explaining a vapor deposition polymerization apparatus. Fig. 1 illustrates a thin-film electroluminescent device of the first aspect of the present invention. The thin-film electroluminescent device 10 comprises a multi-layered structure in which an electroluminescent layer 3 is sandwiched between a negative electrode 1 and a positive electrode 2. The negative electrode 1 is preferably one exhibiting a high efficiency of electron injection into the electroluminescent layer 3 and permitting repeated injections of electrons in the electroluminescent layer 3. The negative electrode 1 is composed of, for example, Mg, In, Ca, Al, Li, Sm or MgAg. The positive electrode 2 is preferably one exhibiting a high efficiency of hole (positive charge) injection into the electroluminescent layer 3 and permitting repeated injections of holes in the electroluminescent layer 3. The positive electrode 2 is composed of, for example, ITO, Pt or Au. At least one of the negative electrode 1 and the positive electrode 2 is transparent, through which irradiation of light emitted in the electroluminescent layer 3 can be transmitted. Generally, either the negative electrode 1 or the positive electrode 2 is formed on a transparent plate of, for example, glass or a polymer film. For example, when the positive electrode 2 is composed of ITO, the ITO electrode is deposited in the form of a thin film on a transparent plate of, for example, glass or a polymer film. The thin-film electroluminescent device 10 illustrated in Fig. 1 comprises an electroluminescent layer 3 composed of a thin film of a polyoxadiazole derivative, formed by the vapor deposition polymerization process, represented by formula (I). The thickness of the film is generally from 20 to 200 nm (200 to 2000 Å), preferably from 30 to 80 nm (300 to 800 Å). The thin film of the polyoxadiazole derivative of formula (I) contains oxadiazole rings, which themselves have electroluminescent properties. Therefore, in formula (I), X 0 and Y 0 are not limited as long as these are divalent organic groups. For ensuring higher electroluminescence efficiency of the thin-film electroluminescent device 10, it is desirable that both of X 0 and Y 0 be electroluminescent divalent organic groups, or that either of X 0 and Y 0 be an electroluminescent divalent organic group, provided that the other is a divalent organic group capable of electron injection and transport, or provided that the other is a divalent organic group capable of hole injection and transport. Appropriate species of X 0 and Y 0 are selected depending on, for example, the type of the material composing the negative electrode 1 and/or the positive electrode 2, or the wavelength (color) of light emitted from the electroluminescent device layer 3. Electroluminescence from the thin-film electroluminescent device 10 is believed to occur as follows. When a voltage is applied between the negative electrode 1 and the positive electrode 2 of the thin-film electroluminescent device 10, electrons are injected from the negative electrode 1 into the electroluminescent layer 3 and migrate through the electroluminescent layer 3, while holes are injected into the electroluminescent layer 3 and migrate through the electroluminescent layer 3. The above electrons and holes are recombined in the electroluminescent layer 3. In the course of the recombination, the electroluminescent layer 3 emits light. Fig. 2 illustrates a first form of a thin-film electroluminescent device according to the second aspect of the present invention. This thin-film electroluminescent device 10 has an electron injecting/transporting layer 4 interposed between the negative electrode 1 and the electroluminescent layer 3 of the thin-film electroluminescent device 10 as illustrated in Fig. 1. The electron injecting/transporting layer 4 increases the efficiency of electron injection from the negative electrode 1 in the electroluminescent device layer 3. In the thin-film electroluminescent device 10 illustrated in Fig. 2, at least one of the electroluminescent layer 3 and the electron injecting/transporting layer 4 is composed of a thin film of the above polyoxadiazole derivative of formula (I). In the thin-film electroluminescent device 10 illustrated in Fig. 2, if one of the electroluminescent layer 3 and the electron injecting/transporting layer 4 is composed of a thin film of the polyoxadiazole derivative of formula (I), the other may be composed of a conventional material. However, it is preferred that both of the electroluminescent layer 3 and the electron injecting/transporting layer 4 be composed of thin films of the polyoxadiazole derivative of formula (I). In the formation of both of the electroluminescent layer 3 and the electron injecting/transporting layer 4 from thin films of the polyoxadiazole derivative of formula (I), the electroluminescent layer 3 and the electron injecting/transporting layer 4 are composed of a polyoxadiazole derivative exhibiting excellent electroluminescence and a polyoxadiazole derivative which has excellent capability of electron injection and transport, respectively. In particular, it is preferred that the electroluminescent layer 3 comprises a polyoxadiazole derivative of formula (I) and that the electron injecting/transporting layer 4 comprises a polyoxadiazole derivative of formula: wherein X 0 , Y 0 and n are as defined above. The electroluminescent layer 3 and the electron injecting/transporting layer 4 are preferably composed of different materials. Therefore, at least one of X 0 and Y 0 in formula (II) is preferably different from both of X 0 and Y 0 in formula (I). The type of the polyoxadiazole derivative comprising the electroluminescent layer 3 is selected in the same manner as in the electroluminescent layer 3 of the thin-film electroluminescent device 10 illustrated in Fig. 1. The type of the polyoxadiazole derivative composing the electron injecting/transporting layer 4, i.e., the species of X 0 and Y 0 in formula (II), are selected in the following manner. In formula (II), X 0 and Y 0 are not particularly limited as long as they are divalent organic groups and either has a capability of electron injection and transport. However, it is preferred that both be divalent organic groups capable of electron injection and transport, or that the one be an electroluminescent divalent organic group while the other is a divalent organic group capable of electron injection and transport. In formulae (I) and (II), appropriate species of X 0 and Y 0 are selected depending on, for example, the type of the material composing the negative electrode 1 and/or the positive electrode 2, or the wavelength (color) of light emitted from the electroluminescent layer 3. When the electron injecting/transporting layer 4 is composed of a thin film of the polyoxadiazole derivative of formula (I), the thickness of the electron injecting/transporting layer 4 is generally 5 to 200 nm (50 to 2000 Å), preferably 5 to 50 nm (50 to 500 Å). Fig. 3 illustrates a second form of a thin-film electroluminescent device according to the second aspect of the present invention. This thin-film electroluminescent device 10 has a hole injecting/transporting layer 5 interposed between the positive electrode 2 and the electroluminescent layer 3 of the thin-film electroluminescent device 10 as illustrated in Fig. 1. The hole injecting/transporting layer 5 increases the efficiency of hole injection from the positive electrode 2 in the electroluminescent layer 3. In the thin-film electroluminescent device 10 illustrated in Fig. 3, at least one of the electroluminescent layer 3 and the hole injecting/transporting layer 5 is composed of a thin film of the above polyoxadiazole derivative of formula (I). In the thin-film electroluminescent device 10 illustrated in Fig. 3, if one of the electroluminescent layer 3 and the hole injecting/transporting layer 5 is composed of a thin film of the polyoxadiazole derivative of formula (I), the other may be composed of a conventional material. However, it is preferred that both of the electroluminescent layer 3 and the hole injecting/transporting layer 5 be composed of thin films of the polyoxadiazole derivative of formula (I). In the formation of both of the electroluminescent layer 3 and the hole injecting/transporting layer 5 from thin films of the polyoxadiazole derivative of formula (I), the electroluminescent layer 3 and the hole injecting/transporting layer 5 are composed of a polyoxadiazole derivative exhibiting excellent electroluminescence and a polyoxadiazole derivative which has excellent capability of hole injection and transport, respectively. In particular, it is preferred that the electroluminescent layer 3 comprises a polyoxadiazole derivative of formula (I) and that the hole injecting/transporting layer 5 comprises a polyoxadiazole derivative of formula: wherein X 0 , Y 0 and n are as defined above. The electroluminescent layer 3 and the hole injecting/transporting layer 5 are preferably composed of different materials. Therefore, at least one of X 0 and Y 0 in formula (II') is preferably different from both of X 0 and Y 0 in formula (I). The type of the polyoxadiazole derivative comprising the electroluminescent layer 3 is selected in the same manner as in the electroluminescent layer 3 of the thin-film electroluminescent device 10 illustrated in Fig. 1. The type of the polyoxadiazole derivative composing the hole injecting/transporting layer 5, i.e., the species of X 0 and Y 0 in formula (II'), are selected in the following manner. In formula (II'), X 0 and Y 0 are not particularly limited as long as they are divalent organic groups and either has a capability of hole injection and transport. However, it is preferred that both be divalent organic groups capable of hole injection and transport, or that the one be an electroluminescent divalent organic group while the other is a divalent organic group capable of hole injection and transport. In formulae (I) and (II'), appropriate species of X 0 and Y 0 are selected depending on, for example, the type of the material composing the negative electrode 1 and/or the positive electrode 2, or the wavelength (color) of light emitted from the electroluminescent layer 3. When the hole injecting/transporting layer 5 is composed of a thin film of the polyoxadiazole derivative of formula (I), the thickness of the hole injecting/transporting layer 5 is generally 5 to 200 nm (50 to 2000 Å), preferably 5 to 50 nm (50 to 500 Å). Fig. 4 illustrates a third form of a thin-film electroluminescent device according to the second aspect of the present invention. This thin-film electroluminescent device 10 has an electron injecting/transporting layer 4 interposed between the negative electrode 1 and the electroluminescent layer 3 of the thin-film electroluminescent device 10 and further a hole injecting/transporting layer 5 interposed between the positive electrode 2 and the electroluminescent layer 3 of the thin-film electroluminescent device 10 as illustrated in Fig. 1. In the thin-film electroluminescent device 10 illustrated in Fig. 4, at least one, preferably all, of the electroluminescent layer 3, the electron injecting/transporting layer 4 and the hole injecting/transporting layer 5 are composed of thin films of the above polyoxadiazole derivative of formula (I). In the formation of all of the electroluminescent layer 3, the electron injecting/transporting layer 4 and the hole injecting/transporting layer 5 from thin films of the polyoxadiazole derivative of formula (I), different polyoxadiazole derivatives are preferably used for the layers. That is, it is preferred that the electroluminescent layer 3, the electron injecting/transporting layer 4 and the hole injecting/transporting layer 5 be composed of a polyoxadiazole derivative exhibiting excellent electroluminescence, a polyoxadiazole derivative which has excellent capability of electron injection and transport and a polyoxadiazole derivative which has excellent capability of hole injection and transport, respectively. With respect to the above polyoxadiazole derivatives, suitable species are selected depending on, for example, the type of the material composing the negative electrode 1 and/or the positive electrode 2, or the wavelength (color) of light emitted from the electroluminescent layer 3. With respect to the polyoxadiazole derivatives for composing the electroluminescent layer 3 of the thin-film electroluminescent device 10 illustrated in Fig. 4, suitable species are selected as with respect to the polyoxadiazole derivatives for composing the electroluminescent layer 3 of the thin-film electroluminescent device 10 illustrated in Fig. 1. Further, with respect to the polyoxadiazole derivatives for composing the electron injecting/transporting layer 4, suitable species are selected as with respect to the polyoxadiazole derivatives for composing the electron injecting/transporting layer 4 of the thin-film electroluminescent device 10 illustrated in Fig. 2. Still further, with respect to the polyoxadiazole derivatives for composing the holeinjecting/transporting layer 5, suitable species are selected as with respect to the polyoxadiazole derivatives for composing the hole injecting/transporting layer 5 of the thin-film electroluminescent device 10 illustrated in Fig. 3. All the thin-film electroluminescent devices 10 illustrated in Figs. 1 to 4 individually have the electroluminescent layer 3 between the electrodes 1 and 2. In the thin-film electroluminescent devices 10, at least one of the negative electrode 1 and the positive electrode 2 is transparent. When a voltage is applied between the electrodes 1 and 2, the electroluminescent layer 3 emits light, which is transmitted through the transparent electrode 1 and/or electrode 2. In the thin-film electroluminescent devices illustrated in Figs. 1 to 4, a thin film of the polyoxadiazole derivative of formula (I), formed according to the vapor deposition polymerization process is disposed as the electroluminescent layer 3 between the electrodes 1 and 2, at least one of which is transparent, and the thin film is disposed as the electron injecting/transporting layer 4 or the hole injecting/transporting layer 5 between the electrode 1 or 2 and the electroluminescent layer 3. That is, in the present invention, at least one of the electroluminescent layer 3, the electron injecting/transporting layer 4 and the hole injecting/transporting layer 5 is composed of a thin film of the polyoxadiazole derivative of formula (I). The polyoxadiazole derivative composing this thin film contains oxadiazole rings, which themselves have electroluminescent properties. Therefore, X 0 and Y 0 in formula (I) are not limited as long as these are divalent organic groups. However, if at least one of X 0 and Y 0 has capability of electroluminescence and/or capability of charge injection and transport (at least one of capability of electron injection and transport and capability of hole injection and transport) the thin-film electroluminescent device 10 can have improved electroluminescence efficiency. From this viewpoint, it is preferred that at least one of the above X 0 and Y 0 have the capability of electroluminescence and/or capability of charge injection and transport. For example, when the electron injecting/transporting layer 4 is composed of the polyoxadiazole derivative of formula (I), it is preferred that at least one of X 0 and Y 0 be selected from the following divalent organic groups. The above groups may be mutually bonded directly or through a divalent bonding group, such as -CH 2 -, -SiH 2 -, -O- or -S-. Of these divalent bonding groups, the hydrogen atoms of -CH 2 - and -SiH 2 - may each be substituted by an alkyl or aryl group. Each of R 1 to R 6 independently is a group selected from hydrogen and halogen atoms and cyano, alkyl, aralkyl and alkyloxy groups. Of the above groups, preferred divalent organic groups having excellent capability of charge injection and transport include groups (E-1) to (E-3), (E-5), (E-7), (E-8), (E-11), (E-21), (E-31), (E-34), (E-35), (E-56), (E-58), (E-60) and (E-70), and especially m-phenylene, p-phenylene, 4,4'-biphenyldiyl and 2,6-pyridinediyl groups. Among them, a p-phenylene group is especially preferred. The electron injecting and transporting layer 4 composed of the polyoxadiazole derivative of formula (I) may comprise electron injecting/transporting additives such as diphenoquinone and fluorenone derivatives disclosed in Chem. Mater., Vol. 3 (1991) pp. 709-714 and J. Imag. Sci., Vol. 29, No. 2 (1985) pp. 69-72, generally in an amount of from 0.01 to 80 % by mole, preferably from 1 to 60 % by mole, per repeating unit of the polyoxadiazole derivative of formula (I). When the hole injecting/transporting layer 5 is composed of the polyoxadiazole derivative of formula (I), each of X 0 and Y 0 independently is a divalent organic group. At least one of X 0 and Y 0 is a group derived from the following compounds: tertiary aromatic amine and porphyrin compounds disclosed in US-A-472,0432 (JP-A-88/295,695), and tertiary aromatic amine compounds disclosed in GB-A-1577237 (JP-A-78/27033), GB-A-1603137 (JP-A-79/58445), GB-A-2023298 (JP-A-79/149634), JP-A-79/64299, US-A-4233384 (JP-A-80/144250), US-A-4273846 (JP-A-81/119132), US-A-4588666 (JP-A-86/295558) and JP-A-86/98303. Examples of groups suitable for use in the hole injecting/transporting layer 5 are as follows: The above groups may be mutually bonded directly or through a divalent bonding group, such as -CH 2 -, -SiH 2 -, -O- or -S-. Of these divalent bonding groups, the hydrogen atoms of -CH 2 - and -SiH 2 - may each be substituted by an alkyl or aryl group. Each of R 7 and R 8 independently is a group selected from hydrogen and halogen atoms and cyano, alkyl, aralkyl and alkyloxy groups. Of the above groups, preferred divalent organic groups having excellent capability of charge injection and transport include groups (H-1), (H-2), (H-5), (H-6), (H-14), (H-16), (H-19), (H-22) and (H-24). The hole injecting/transporting layer 5 composed of the polyoxadiazole derivative of formula (I) may optionally comprise a hole injecting/transporting additive such as 4,4',4"-tris(N,N-diphenylamino)triphenylamine, 4,4',4"-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine and other triphenylamine derivatives disclosed in Chem. Lett., 1989, p.1145, generally in an amount of from 0.01 to 80 % by mole, preferably from 1 to 50 % by mole, per repeating unit of the polytoxadiazole derivative of formula (I). When at least one of the divalent organic groups represented by X 0 and Y 0 is the divalent group capable of electron injection and transport or the divalent group capable of hole injection and transport, the thin film composed of the polyoxadiazole derivative of formula (I) not only has the capability of charge injection and transport but also is itself capable of emitting fluorescence. For example, when X 0 and Y 0 in formula (I) represent a 1,4-phenylene group and a 1,3-phenylene group, respectively, the polymeric thin film composed of the polyoxadiazole derivative of formula (I) emits blue fluorescence maximized at a wavelength of 410 nm. On the other hand, when both X 0 and Y 0 in formula (I) represent 1,4-phenylene groups, the polymeric thin film composed of the polyoxadiazole derivative of formula (I) emits blue fluorescence maximized at a wavelength of 450 nm. Thus, the polymeric thin film composed of the polyoxadiazole derivative of formula (I) can cause the thin-film electroluminescent device 10 to emit fluorescence having a wavelength of 400 to 600 nm, depending on the combinations of X 0 and Y 0 in formula (I). Further, when at least one of the divalent organic groups represented by X 0 and Y 0 in formula (I) is selected from the following divalent groups which have electroluminescent properties, the electroluminescent device efficiency of the polyoxadiazole derivative of formula (I) is remarkably improved. Among these groups, the group (L-1) is especially preferred. The electroluminescent layer 3 composed of the polyoxadiazole derivative of formula (I) may optionally comprise a fluorescent dye or pigment such as coumarin 343, NK 757, DCM and NK 2929 and an aluminum quinolinol complex known as laser dyes, generally in an amount of from 0.01 to 10 % by mole, preferably from 0.01 to 5 % by mole, per repeating unit of the polyoxadiazole derivative of formula (I). When a voltage is applied between the electrodes 1 and 2 of the thin-film electroluminescent device 10 having the electroluminescent layer 3 comprising the polyoxadiazole derivative of formula (I) and the above fluorescent dye or pigment, the thin-film electroluminescent device 10 emits fluorescence characteristic of the fluorescent dye or pigment. For example, when coumarin 343, an aluminum quinolinol complex, Nk757 and DCM are individually used as a fluorescent dye, the electroluminescent device 10 emits bluish-green, green, yellow and red fluorescence, respectively. Thus, according to the present invention, the color of light emitted by the electroluminescent device 10 can be controlled by causing the electroluminescent layer 3 composed of the polyoxadiazole derivative of formula (I) to contain a particular fluorescent dye or pigment. In the molecule of the polyoxadiazole derivative of formula (I), the oxadiazole rings are bonded alternatively with X 0 s and Y 0 s. When both of X 0 and Y 0 are π-electron conjugated groups, it is expected that a continuous conjugation of π-electrons can be provided along the principal chain of the molecule which thereby has excellent electron-conductivity. Therefore, both of X 0 and Y 0 are preferably π-electron conjugated groups. In the polyoxadiazole derivative of formula (I), a carbonyl group, being a trap of a carrier (electron or hole) as in the polyimide of formula (IX), does not form part of the principal chain of the polyoxadiazole. Therefore, the thin film composed of the polyoxadiazole derivative of formula (I) is expected to have excellent electron conductivity as compared with the conventional thin film composed of the polyimide of formula (IX). In the present invention, preferably at least one, still preferably both of the electron injecting /transporting layer 4 and the hole injecting/transporting layer 5 are silicon-atom containing polymeric compounds. In particular, when the electrode 1 or 2 is provided in the form of a thin film on a glass plate, or when the electrode 1 or 2 is a metal electrode, the formation of the above charge injecting/transporting layer 4 or 5 comprising a silicon-atom containing polymeric compound between the electrode 1 or 2 and the electroluminescent layer 3 improves the adhesion between the electrode 1 or 2 and the electroluminescent layer 3. The silicon-atom containing polymeric compound for use in the charge injecting/transporting layer 4 or 5 preferably has a capability of charge injection and transport. Especially, in particular, the electroluminescent layer 3 of each of the thin-film electroluminescent devices 10 illustrated in Figs. 2 to 4 is composed of a thin film of the polyoxadiazole derivative of formula (I), and it is preferred that at least one of the charge injecting/transporting layers 4 and 5 be composed of a silicon-atom containing polyoxadiazole of formula: wherein each of X 1 and Y 1 independently represents a divalent organic group and at least one of which has a silicon atom, and n is an integer of 2 or greater. The divalent organic groups having a silicon atom include divalent organic groups formed by mutual bonding through a silicon-atom containing divalent bonding group, such as -SiH 2 -, of the divalent organic groups capable of electron or hole injection and transport mentioned above as examples of the divalent organic groups comprising a silicon atom, for example, When the thin film of the silicon-atom containing polyoxadiazole of formula (III) is formed according to the vapor deposition polymerization process, the thin film constitutes part of the thin film of the polyoxadiazole derivative of formula (I). Thus, when the thin film of the silicon-atom containing polyoxadiazole of formula (III) is formed according to the vapor deposition polymerization process as the charge injecting/transporting layer (electron injecting/transporting layer 4 and/or hole injecting/transporting layer 5) of each of the thin-film electroluminescent devices 10 illustrated in Figs. 2 to 4, the electroluminescent layer 3 may be composed of the conventional electroluminescent material. Preferably, the charge injecting/transporting layer (electron injecting/transporting layer 4 and/or hole injecting/transporting layer 5) composed of the silicon-atom containing polymeric compound further comprises a polycondensed polymeric compound containing no silicon atoms, in which the content of the silicon-atom containing polymeric compound decreases from the electrode 1 or 2 towards the electroluminescent layer 3 continuously as shown by a broken line in Fig. 5 or stepwise as shown by a dot-dashed line in Fig. 5 while the content of the polycondensed polymeric compound containing no silicon atoms increases from the electrode towards the electroluminescent layer continuously as shown by a broken line in Fig. 5 or stepwise as shown by a dot-dashed line in Fig. 5. The polycondensed polymeric compound containing no silicon atoms preferably has a capability of charge injection and transport. The polycondensed polymeric compound containing no silicon atoms includes, for example, polymeric compounds such as polyimides, polyamides, polyamide-imides, polyureas and polyazomethines (see Polymer Preprints Japan, Vol. 136, p1475 and p3021 (1987)) and polymeric compounds having a polyoxadiazole structure of formula: wherein each of X 2 and Y 2 independently represents a divalent organic group which does not have a silicon atom, and n is an integer of 2 or greater. The X 2 s and Y 2 s in formula (IV) include divalent organic groups capable of electron or hole injection and transport mentioned above as examples of X 0 s and Y 0 s contained in the polyoxadiazole derivative of formula (I) and divalent organic groups formed by mutual bonding thereof through a divalent bonding group containing no silicon atoms, such as -CH 2 -, -O- and -S-. Of these divalent bonding groups, the hydrogen atoms of -CH 2 - may each be substituted by an alkyl or aryl group. In particular, when the silicon-atom containing polymeric compound is the silicon-atom containing polyoxadiazole of formula (III), it is preferred that the polycondensed polymeric compound containing no silicon atoms be the polyoxadiazole of formula (IV). When both the silicon-atom containing polyoxadiazole of formula (III) and the polyoxadiazole containing no silicon atoms of formula (IV) are produced by the vapor deposition polymerization process, the thus formed thin polyoxadiazole film constitutes part of the thin film of the polyoxadiazole derivative of formula (I). Thus, when the charge injecting/transporting layer (electron injecting/transporting layer 4 and/or hole injecting/transporting layer 5) of each of the thin-film electroluminescent devices 10 illustrated in Figs. 2 to 4 is composed of the above thin polyoxadiazole film, the electroluminescent layer 3 may be composed of the conventional electroluminescent material. It is preferred, however, that the electroluminescent layer 3 be the same as the thin film of the polyoxadiazole containing no silicon atoms of formula (IV). When the polyoxadiazole containing no silicon atoms included in the charge injecting/transporting layer is the same as the polyoxadiazole composing the electroluminescent layer 3, the charge injecting/transporting layer is continuously unified with the electroluminescent layer 3 which thereby creates resistance to peeling between the charge injecting/transporting layer and the electroluminescent layer 3. In each of the thin-film electroluminescent devices 10 illustrated in Figs. 1 to 4, a protective film, such as an antioxidant film, may be provided either to cover the surfaces of the negative electrode 1 and the positive electrode 2 or to seal the whole body of the device 10. The protective film provided on the negative electrode 1 and the positive electrode 2 increases the stability thereof, thereby improving the practicability and durability of the thin-film electroluminescent device 10. The protective film may be composed of a metal exhibiting a high work function, amorphous silica, germanium monoxide, silicon monoxide, an epoxy resin, a silicone resin or a fluorinated resin. The above-mentioned thin-film electroluminescent device of the present invention can be produced by: i) forming an electrode 1 or 2 on a plate; ii) optionally forming a first charge injecting/transporting layer 4 or 5 on the electrode 1 or 2 by the vapor deposition polymerization process; iii) forming an electroluminescent layer 3 on either the electrode 1 or 2 or the first charge injecting/transporting layer 4 or 5 by the vapor deposition polymerization process; iv) optionally forming a second charge injecting/transporting layer 4 or 5 (for example, the second charge injecting/transporting layer is a hole injecting/transporting layer 5 when the first charge injecting/transporting layer is an electron injecting/transporting layer 4) capable of injection and transport of charges opposite to those injected in and transported by the first charge injecting/transporting layer 4 or 5 on the electroluminescent layer 3 by the vapor deposition polymerization process; v) forming a counter electrode 1 or 2 (when the formed (for example, in step i) electrode is a negative electrode 1, the counter electrode is a positive electrode 2) on either the electroluminescent layer 3 or the second charge injecting/transporting layer 4 or 5; and vi) optionally providing a sealing layer for the electroluminescent device on the counter electrode. In the present invention, the electroluminescent layer 3 and/or the charge injecting/transporting layer (electron injecting/transporting layer 4 and/or hole injecting/transporting layer 5) is composed of a thin film of the polyoxadiazole derivative of formula (I), which is produced by the vapor deposition polymerization process. In the present invention, in the formation of the thin film of the polyoxadiazole derivative of formula (I) by the vapor deposition polymerization process, use is made of, as monomers for condensation polymerization, a dicarboxylic acid of formula: wherein X 0 represents a divalent organic group which may comprise a silicon atom, and each of Z 1 and Z 2 independently represents a halogen atom or an organic group represented by the formula JK- in which J represents a monovalent organic group having an organic ring structure and K represents an atom selected from nitrogen, oxygen and sulfur, provided that when K is a nitrogen atom, the nitrogen atom may constitute part of the organic ring structure of J, and a dihydrazide compound of formula: wherein Y 0 represents a divalent organic group which may comprise a silicon atom. X 0 in formula (V) and Y 0 in formula (VI) represent the same groups as represented by X 0 s and Y 0 s in formula (I), and specific species thereof are selected in the same manner as for those of X 0 s and Y 0 s in formula (I). Z 1 and Z 2 in formula (V) may represent halogen atoms, such.as chlorine. However, if the electroluminescent layer 3 and/or the charge injecting/transporting layer (electron injecting/transporting layer 3 and/or hole injecting/transporting layer 5) is composed of the thin film of the polyoxadiazole derivative formed by the vapor deposition polymerization of the dicarboxylic acid derivative of the formula (V) in which at least one of Z 1 and Z 2 is a halogen atom and the dicarbodihydrazide compound of the formula (VI), the halogen atom positioned at the terminal of the dicarboxylic acid derivative reacts with the -NH 2 group at the terminal of the dihydrazide compound which incidentally produces a hydrogen halide which is a strong acid. This hydrogen halide possibly corrodes the electrode 1 and/or 2 in any of the thin-film electroluminescent devices 10 illustrated in Figs. 1 to 4 or possibly corrodes the frame of the apparatus for vapor deposition used in the vapor deposition polymerization itself even if the electrode 1 and/or 2 is not corroded. When there is possibility of such occurrences, it is preferred that both Z 1 and Z 2 in formula (V) be organic groups of formula JK-. When the electroluminescent layer 3 and/or the charge injecting/transporting layer is composed of the thin film of the polyoxadiazole derivative formed by the vapor deposition polymerization of the dicarboxylic acid derivative of formula (V) in which both of Z 1 and Z 2 are organic groups of formula JK- and the dicarbodihydrazide compound of formula (VI), the by-products formed by the polymerization of the above monomers are organic compounds of formula JKH, which are not strong acids, unlike the above hydrogen halide. Therefore, there is no corrosion of the electrode 1 and/or 2 in any of the thin-film electroluminescent devices 10 illustrated in Figs. 1 to 4 and further corrosion of the apparatus for vapor deposition used in the vapor deposition polymerization. Especially preferred groups Z 1 and Z 2 in formula (V) are as follows: For example, when Z 1 and Z 2 in formula (V) represent 2-benzoxazoylthio, 2-benzothiazoylthio, 2-benzothiazolyloxy, 2-benzisoxazoyloxy, 1-benzotriazoyloxy or 2,4-dinitrophenoxy groups, these groups react with the terminal group -NH 2 in formula (II) in the vapor deposition polymerization to thereby form corresponding by-products, i.e., 2-mercaptobenzoxazole, 2-mercaptobenzothiazole, 2-hydroxybenzothiazole, 2-hydroxyisobenzoxazole, 1-hydroxybenzotriazole or 2,4-dinitrophenol. These by-products do not corrode the electrode of the organic electroluminescent device or the apparatus for vapor deposition, unlike the hydrogen halide occurring in the vapor deposition polymerization of an acid halide and a dihydrazide. In the present invention, the thin film of the polyoxadiazole derivative of formula (I) is produced by the following sequence of steps. (a) First, a plate 22 to be coated, for example a plate with an electrode of, for example, ITO, is disposed in a vapor deposition chamber 21 of a vapor deposition apparatus 20 as shown in Fig. 6 in an arrangement such that a vapor deposition film is formed on the ITO electrode (surface to be coated). (b) The above two monomers A and B for condensation polymerization are put in separate vapor sources 23a, 23b in the vapor deposition apparatus 20. (c) The inside of the vapor deposition chamber 21 is evacuated to a pressure of generally 10 -2 Pa or lower, preferably 10 -3 Pa or lower. (d) During the evacuation of the inside of the vapor deposition chamber 21 to the above pressure, the temperature of the surface to be coated, e.g., the ITO electrode side of the plate with the ITO electrode, is adjusted to -50 to 200 °C, preferably 20 to 100 °C. (e) When the pressure of the inside of the vapor deposition chamber 21 reaches the above predetermined value, the vapor sources 23a, 23b each having the monomers A and B for condensation polymerization placed therein are regulated to a temperature at which at an evaporation speed of not less than 10 -10 mol/sec·cm 2 , preferably not less than 10 -8 mol/sec·cm 2 , each of the monomers A, B is evaporated under the pressure. Such a temperature is generally from -10 to 500 °C, preferably from 40 to 400 °C, more preferably from 70 to 300 °C, especially preferably from 100 to 250 °C. If at least one of the monomers for condensation polymerization is evaporated at temperatures lower than room temperature, for example -10 °C, under the predetermined pressure, the vapor source having the monomer placed therein or the place in which the vapor deposition apparatus is set is cooled to a temperature of, for example, lower than -10 °C prior to the heating of the vapor source in order to prevent the evaporation of the monomer before the predetermined pressure is reached. The pressure inside the vapor deposition chamber and the heating temperature for evaporating the monomers for polymerization are generally regulated so as to achieve the evaporation of the dicarboxylic acid derivative of formula (V) and the dihydrazide compound of formula (VI) in a molar ratio of 1:1 to 1:30, preferably 1:1 to 1:20, and also the formation of a vapor deposition film at a rate of 0.01 to 1 nm/s (0.1 to 10 Å/sec), preferably 0.1 to 0.4 nm/s (1 to 4 Å/sec). Copolymerization proceeds on the coated surface. (f),Thus, a vapor deposition film having a thickness of about 5 to 200 nm (50 to 2000 Å) is formed. This vapor deposition film is heated at 100 to 400 °C, preferably 200 to 300 °C, for 1 to 240 min, preferably 10 to 120 min in a vapor or an inactive gas. As a result, a polycarbohydrazide as a precursor from the monomers is converted to the polyoxadiazole derivative of formula (I), and a thin film thereof is formed on the coated surface, such as the ITO electrode of the plate with the ITO electrode. The polymerization degree of the thus obtained polyoxadiazole derivative corresponds to the value of n in formula (I), and is an integer of 2 or greater, preferably 10 or greater, more preferably 100 or greater. The polycarbohydrazide as a precursor of the polyoxadiazole of formula (I) is soluble in organic polar solvents, such as dimethylformamide (DMF), dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), N-methylpyrrolidone and pyridine. By contrast, the polyoxadiazole of formula (I) is sparingly soluble in concentrated sulfuric acid. Thus, conveniently, the polymerization degree of the polycarbohydrazide is measured by the use of the above organic solvent, and the polymerization degree n of the polyoxadiazole of formula (I) is presumed therefrom. When it is desired to incorporate additives, such as those capable of electron injection and transport, capable of hole injection and transport and capable of electroluminescence mentioned above, in the thin film of the polyoxadiazole of formula (I), this can be accomplished by placing the additives in a vapor source separate from those for the carboxylic acid derivative of formula (V) and the dihydrazide compound of formula (VI) in step (b) and then carrying out steps (c) through (f) as described above. The vapor deposition film formed in step (e) can precisely be controlled to provide a stoichiometrically excellent thin film of the polyoxadiazole derivative of formula (I) by regulating the pressure inside the vapor deposition chamber to 10 -2 - 10 -4 Pa in step (c) and by regulating the temperatures of the vapor sources having the monomers for condensation polymerization placed therein to 40 - 400 °C in step (e) so that the monomers placed in the vapor sources are evaporated at a rate of at least 10 -10 mol/sec·cm 2 . As the monomers for condensation polymerization exhibiting the above characteristics, there may be mentioned dicarboxylic acid derivatives of formula (V), each having as X 0 a divalent organic group with 2 to 50 carbon atoms containing an alkylene group or an aromatic ring, and dihydrazide compounds of formula (VI), each having as Y 0 the same divalent organic group as had by X 0 . In particular, examples of suitable X 0 and Y 0 groups are as follows. In the above formulae, each of R 9 to R 13 independently represents an unsubstituted or substituted alkyl group, allyl group, aryl group or aralkyl group. Each of B 2 to B 5 independently represents -CH 2 -, -SiH 2 -, -O- or -S-. Of these, the hydrogen atoms of -CH 2 - and -SiH 2 - may each be substituted by an alkyl or aryl group. In particular, when X 0 in formula (V) and Y 0 in formula (VI) are selected from the above examples of groups and Z 1 and Z 2 in formula (V) are selected from the above examples of organic groups, and when the pressure inside the vapor deposition chamber is 10 -2 - 10 -4 Pa at the time of vapor deposition and the temperatures of the vapor sources having the respective monomers for condensation polymerization placed therein are 40 - 400 °C, the formation of the vapor deposition film in step (e) can precisely be controlled without the incidental occurrence of strong acid hydrogen halides so that a desirable thin film of the polyoxadiazole derivative of formula (I) can be obtained. When the above-mentioned additives, such as those capable of electron injection and transport, capable of hole injection and transport and capable of electroluminescence, are to be introduced in the thin film of the polyoxadiazole derivative of formula (I), the additives are placed in a vapor source separate from those for the carboxylic acid derivative of formula (V) and for the dihydrazide compound of formula (VI) in step (b) and thereafter steps (c) to (f) are carried out in the same manner as described above. Thus, the additives and the monomers undergo simultaneous vapor deposition, so that a desired thin film of the polyoxadiazole derivative of formula (I) containing the additives can be obtained. When the charge injecting/transporting layer (electron injecting/transporting layer 4 and/or hole injecting/transporting layer 5) of each of the thin-film electroluminescent devices 10 illustrated in Figs. 2 to 4 is to be composed of a layer comprising a silicon-atom containing polymeric compound, the formation thereof is conducted in an appropriate manner complying with the properties of the silicon-atom containing polymeric compound. For example, when the silicon-atom containing polymeric compound is the polyoxadiazole derivative of formula (III), the thin film thereof is produced in the same manner as in the production of the thin film of the polyoxadiazole derivative of formula (I). When the charge injecting/transporting layer (electron injecting/transporting layer 4 and/or hole injecting/transporting layer 5) of each of the thin-film electroluminescent devices 10 illustrated in Figs. 2 to 4 comprises a silicon-atom containing polymeric compound and a polycondensed polymeric compound containing no silicon atoms, the content of the silicon-atom containing polymeric compound decreasing from the electrode toward the electroluminescent layer and the content of the polycondensed polymeric compound containing no silicon atoms increasing from the electrode toward the electroluminescent layer, the gradients of the contents of these compounds are created in an appropriate manner complying with the properties of the compounds. For example, when the layer with the above content gradients is composed of polyoxadiazole derivatives, it is preferred that the carboxylic acid derivative of formula (V) and the dihydrazide compound of formula (VI) (provided that at least one, preferably both, of X 0 in formula (V) and Y 0 in formula (VI) contains silicon atoms) be used as the monomers for polymerization to obtain the silicon-atom containing polyoxadiazole derivative and subjected to the vapor deposition polymerization, and that the carboxylic acid derivative of formula (V) and the dihydrazide compound of formula (VI) (provided that both X 0 in formula (V) and Y 0 in formula (VI) contain no silicon atoms) be used as the monomers for polymerization to obtain the polyoxadiazole derivative containing no silicon atoms and subjected to the vapor deposition polymerization. When the layer in which the proportion of the polyoxadiazole derivative containing silicon atoms to the polyoxadiazole derivative containing no silicon atom is changed from the electrode 1 or 2 towards the electroluminescent layer 3 is produced, a technique is adopted in which, in the vapor deposition apparatus, the amount of evaporated silane monomer (the carboxylic acid derivative of formula (V) containing silicon atoms and/or the dihydrazide compound of formula (VI) containing a silicon atom) and the amount of evaporated nonsilane monomer (the carboxylic acid derivative of formula (V) containing no silicon atoms and/or the dihydrazide compound of formula (VI) containing no silicon atoms) are changed with the lapse of time. As the method for changing the above evaporation amount with the lapse of time, there may be mentioned a method in which first the silane monomer is evaporated in the vapor deposition chamber under predetermined vacuum conditions and deposited on the electrode 1 or 2, the evaporation of the silane monomer is rapidly discontinued by closing a shutter in front of the vapor source having the monomer placed therein, and then immediately the nonsilane monomer is evaporated. Further, there may be mentioned, for example, a method in which the rate of evaporation of the silane monomer is gradually decreased to finally become nil while the rate of evaporation of the nonsilane monomer is gradually increased. An appropriate method is selected depending on, for example, the type of the plate or the types of monomers. In the present invention, in producing the thin-film electroluminescent devices 10 illustrated in Figs. 1 to 4, it is preferred that all of the steps from formation of an electrode on a plate until the formation of the counter electrode, i.e., steps i) to v), be continuously performed in a single vapor deposition chamber, and that each of steps i) to iv) before the formation of the counter electrode be followed without inflow of outside air into the vapor deposition chamber by the subsequent step. The production of the thin-film electroluminescent device 10 in the above manner prevents the dust present in outside air from adhering to the layers formed between the electrodes 1 and 2 to degrade the thin-film electroluminescent device 10. Also, it prevents the oxygen and moisture in outside air from being adsorbed onto each of the layers formed between the electrodes 1 and 2 to thereby oxidize the electrode 1 and/or 2, the oxidation leading to degradation of the thin-film electroluminescent device 10. The thus obtained electroluminescent device (EL device) of the present invention emits light when a voltage is applied between the electrodes 1 and 2. The applied voltage may be not only a direct current (DC) voltage but also a driving voltage having a waveform such as that of a pulse or chopped wave. When a pulse voltage is applied, the power consumption can markedly be saved as compared with the application of DC voltage. The EL device can also be utilized as a display device by driving the device with the voltage having a specific waveform as mentioned above. Further, the EL device of the present invention can be utilized as a display device by driving through a matrix electrode or a thin film transistor (TFT) electrode having a pattern as the negative electrode 1 (electrode for injecting electrons) or the positive electrode 2 (electrode for injecting holes). The present invention provides a thin-film electroluminescent device which not only has excellent electroluminescence properties and capability of charge injection and transport to exhibit high electroluminescence efficiency but also has excellent durability, such as heat resistance. EXAMPLES The present invention will now be described in further detail in the following Examples. Example 1 Vapor Deposition Polymerization An ITO-coated glass plate having a thickness of 100 nm (1000 Å) (manufactured by Hoya Corp.) was subjected to ultrasonic cleanings successively using acetone, deionized water, plate cleaner (Semicoclean, grade EL, produced by Furuuchi Kagaku), deionized water and isopropyl alcohol (IPA). From boiling isopropyl alcohol, the plate was taken out and dried. The thus dried ITO-coated glass plate was mounted on a temperature-controllable plate holder disposed in a vapor deposition apparatus. Then, 1 g of commercially available terephthaloyl dichloride (produced by Tokyo Kasei) and 1 g of commercially available terephthaloyl dihydrazide (produced by Wako Pure Chemical Industries, Ltd.) as monomers were placed in separate vapor sources in the vapor deposition apparatus. The inside of the vapor deposition apparatus was evacuated to a pressure of 1 x 10 -3 Pa or below by an oil diffusion pump. In the beginning, a shutter disposed in front of the coated plate for isolating the coated plate from the vapor sources was closed, and, while keeping the shutter closed, the vapor sources were heated by the resistance heating method. Conditions were created for ensuring the evaporation of each of the monomers at a rate of 10 -8 to 10 -7 mol/sec·cm 2 , and the shutter in front of the coated plate was opened. Thus, the vapor deposition of the monomers on the ITO-coated plate was carried out. When the thickness of the vapor deposition film on the coated plate reached 80 nm (800 Å) as measured by a quartz resonator film thickness meter, the shutter was again closed. The plate holder was heated to a temperature of 300 °C to thermally treat the vapor deposition film-coated plate for 30 min. This treatment completed the polymerization of the above terephthaloyl dichloride and terephthaloyl dihydrazide. Confirmation of Formation of Polymeric Thin Film of Poly(1,4-phenylene-2,5-oxadiazolylene) The same operation as above was conducted on an Al plate having a thickness of 0.5 mm to thereby form a vapor deposition film (specimen) having a thickness of 1 µm. An FT-IR spectrum of the specimen was taken by the reflection method. The spectrum showed the disappearances of absorption at 3212 cm -1 characteristic of a hydrazide group (N-H stretching vibration) and of the C=O stretching vibration at 1666 cm -1 of a carbonyl group. Further, the spectrum showed absorptions at 1478 and 1536 cm -1 (-C=N- and >C=C< stretching vibrations) and at 1002 and 959 cm -1 C=C-O-C= stretching vibration) ascribed to the oxadiazole ring. Thus, the formation of the oxadiazole ring was confirmed. The thin film was dissolved in concentrated sulfuric acid only, but was not dissolved in organic solvents. Thus, the formation of poly(1,4-phenylene-2,5-oxadiazolylene) by the polymerization of terephthaloyl dichloride and terephthaloyl dihydrazide was confirmed. Preparation of Electroluminescent device and Confirmation of Electroluminescence A counter electrode of an MgAg alloy having a weight ratio of Mg to Ag of 10:1 was formed by codeposition on the polymeric thin film of poly(1,4-phenylene-2,5-oxadiazolylene) (electroluminescent device layer) superimposed on the ITO-coated glass plate. Thus, a thin-film electroluminescent device 10 as illustrated in Fig. 1 was obtained. Using the ITO electrode and the MgAg electrode as a positive electrode 2 and a negative electrode 1, respectively, a direct current voltage of 7 V was applied between the electrodes 1 and 2. The electroluminescent device emits produced blue electroluminescence (EL). Example 2 A polymeric thin film of poly(1,4-phenylene-2,5-oxadiazolylene) was formed on an ITO-coated glass plate according to the vapor deposition polymerization process in the same manner as in Example 1, except that laser dye DCM was evaporated from a vapor source separate from those for the monomers and codeposited on the plate. The rate of evaporation and deposition of DCM was regulated so that the content of DCM relative to the total amount of the deposited monomers in the polymeric thin film was 0.05 % by mole. An MgAg electrode was formed by codeposition on the thus obtained polymeric thin film of poly(1,4-phenylene-2,5-oxadiazolylene) containing 0.05 % by mole of DCM, thereby preparing an electroluminescent device. Thus, a thin-film electroluminescent device 10 as illustrated in Fig. 1 was obtained. Using the ITO electrode as a positive electrode 2 and the MgAg electrode as a negative electrode 1, a direct current voltage of 10 V was applied between the electrodes 1 and 2. The electroluminescent device emits produced red electroluminescence (wavelength of 610 nm). Example 3 A polymeric thin film of poly(1,4-phenylene-2,5-oxadiazolylene) was formed on an ITO-coated glass plate according to the vapor deposition polymerization process in the same manner as in Example 1, except that 4,4',4"-tris[N-(3-methylphenyl)-N-phenylamino]tri-phenylamine as a triphenylamine derivative having a high hole transport capability was evaporated from a vapor source separate from those for the monomers and codeposited on the plate. The rate of evaporation and deposition of the triphenylamine derivative was regulated so that the content of the triphenylamine derivative relative to the total amount of the deposited monomers in the polymeric thin film was 7 % by mole. When the thickness of the vapor deposition film containing the triphenylamine derivative in an amount of 7 % by mole relative to the total amount of the monomers was confirmed to be 30 nm (300 Å) by a quartz resonator film thickness meter, the deposition of the triphenylamine derivative was stopped. Thereafter, the deposition of the monomers only was continued until it was stopped when the total thickness of the vapor deposition film was 80 nm (800 Å). Thermal treatment was conducted under the same conditions as in Example 1 to complete the polymerization reaction. An MgAg electrode was formed by codeposition on the thus obtained polymeric thin film, thereby preparing an electroluminescent device. Thus, a thin-film electroluminescent device 10 as illustrated in Fig. 1 was obtained. Using the ITO electrode as a positive electrode 2 and the MgAg electrode as a negative electrode 1, a direct current voltage was applied between the electrodes 1 and 2. The electroluminescent device emits produced blue electroluminescence. The voltage at which electroluminescence started was lower than that of the electroluminescent device in which the triphenylamine was contained. The electroluminescence brightness of the device at the same voltage was higher, showing an improvement in the electroluminescence efficiency. Example 4 Vapor Deposition Polymerization The same cleaned ITO-coated glass plate as in Example 1 was mounted on a temperature-controllable plate holder disposed in a vapor deposition apparatus. Then, 1 g of commercially available 4,4'-biphenyldicarbonyl dichloride and 1 g of commercially available terephthaloyl dihydrazide (produced by Wako Pure Chemical Industries, Ltd.) as monomers were placed in separate vapor sources in the vapor deposition apparatus. The inside of the vapor deposition apparatus was evacuated to a pressure of 1 x 10 -3 Pa or below by an oil diffusion pump. In the beginning, a shutter disposed in front of the coated plate for isolating the coated plate from the vapor sources was closed, and, while keeping the shutter closed, the vapor sources were heated by the resistance heating method. Conditions were created for ensuring the evaporation of each of the monomers on the coated plate at a rate of 10 -8 to 10 -7 mol/sec.cm 2 , and the shutter in front of the coated plate was opened. Thus, the vapor deposition of the monomers on the ITO-coated plate was carried out. When the thickness of the vapor deposition film on the coated plate reached 80 nm (800 Å) as measured by a quartz resonator film thickness meter, the shutter was again closed. The plate holder was heated to a temperature of 300 °C to thermally treat the vapor deposition film-coated plate for 30 min. This treatment completed the polymerization of the above 4,4'-biphenyldicarbonyl dichloride and terephthaloyl dihydrazide. Confirmation of Formation of Polymeric Thin Film of Poly(4',4-biphenylene-2,5-oxadiazolylene-1,4-phenylene-2,5-oxadiazolylene) The same operation as above was conducted on an Al plate having a thickness of 0.5 mm to thereby form a vapor deposition film (specimen) having a thickness of 1 µm. An FT-IR spectrum of the specimen was taken by the reflection method. The spectrum showed the disappearances of absorption at 3212 cm -1 characteristic of a hydrazide group (N-H stretching vibration) and of the C=O stretching vibration of a carbonyl group at 1666 cm -1 . Further, the spectrum showed absorptions ascribed to the oxadiazole ring (1478, 1536, 1002, 959 cm -1 ). Thus, the formation of the oxadiazole ring was conrimed. The FT-IR spectrum of the thin film obtained by the above vapor deposition polymerization agreed with that of a thin film obtained by the solution polymerization technique. Thus, the formation of poly(4',4-biphenylene-2,5-oxadiazolylene-1,4-phenylene-2,5-oxadiazolylene) by the polymerization of 4,4'-biphenyldicarbonyl dichloride and terephthaloyl dihydrazide was confirmed. Preparation of Electroluminescent Device and Confirmation of Electroluminescence A counter electrode of an MgAg alloy having a weight ratio of Mg to Ag of 10:1 was formed by codeposition on the polymeric thin film of poly(4',4-biphenylene-2,5-oxadiazolylene-1,4-phenylene-2,5-oxadiazolylene) (electroluminescent device layer) superimposed on the ITO-coated glass plate to thereby prepare an electroluminescent device. Thus, a thin-film electroluminescent device 10 as illustrated in Fig. 1 was obtained. Using the ITO electrode as a positive electrode 2 and the MgAg electrode as a negative electrode 1, a direct current voltage of 7 V was applied between the electrodes 1 and 2. The electroluminescent device emits produced blue electroluminescence (EL). Example 5 An electroluminescent device was produced in the same manner as in Example 2, except that poly(4',4-biphenylene-2,5-oxadiazolylene-1,4-phenylene-2,5-oxadiazolylene) was employed in place of the poly(1,4-phenylene-2,5-oxadiazolylene). Thus, a thin-film electroluminescent device 10 as illustrated in Fig. 1 was obtained. Using the ITO electrode as a positive electrode 2 and the MgAg electrode as a negative electrode 1, a direct current voltage of 10 V was applied between the electrodes 1 and 2. The electroluminescent device produced emits red electroluminescence (wavelength of 610 nm). Example 6 An electroluminescent device was produced in the same manner as in Example 3, except that poly(4',4-biphenylene-2,5-oxadiazolylene-1,4-phenylene-2,5-oxadiazolylene) was employed in place of the poly(l,4-phenylene-2,5-oxadiazolylene). Thus, a thin-film electroluminescent device 10 as illustrated in Fig. 1 was obtained. Using the ITO electrode as a positive electrode 2 and the MgAg electrode as a negative electrode 1, a direct current voltage was applied between the electrodes 1 and 2. The electroluminescent device emits produced blue electroluminescence. The voltage at which electroluminescence started was lower than that of the electroluminescent device in which the triphenylamine was contained. The electroluminescence brightness of the device at the same voltage was higher showing an improvement in the electroluminescence efficiency. Example 7 Vapor Deposition Polymerization The same cleaned ITO-coated glass plate plate as in Example 1 was mounted on a temperature-controllable plate holder disposed in a vapor deposition apparatus. Then, 1 g of 5-diphenylamino-isophthaloyl chloride, 1 g of 5-tert-butylisophthaloyl chloride and 1 g of terephthaloyl dihydrazide (produced by Wako Pure Chemical Industries, Ltd.) as monomers were placed in separate vapor sources in the vapor deposition apparatus. The inside of the vapor deposition apparatus was evacuated to a pressure of 1 x 10 -3 Pa or below by an oil diffusion pump. In the beginning, a shutter disposed in front of the coated plate for isolating the coated plate from the vapor sources was closed, and, while keeping the shutter closed, the vapor sources were heated by the resistance heating method. First, conditions were created for ensuring the evaporation of each of 5-diphenylamino-isophthaloyl chloride and terephthaloyl dihydrazide on the coated plate at a rate of 10 -8 to 10 -7 mol/sec·cm 2 , and the shutter in front of the coated plate was opened. Thus, the vapor deposition of the above monomers on the ITO-coated plate was carried out. When the thickness of the vapor deposition film on the coated plate reached 50 nm (500 Å) as measured by a quartz resonator film thickness meter, the shutter was again closed. Subsequently, conditions were created for ensuring the evaporation of each of 5-tert-butylisophthalolyl chloride and terephthaloyl dihydrazide on the coated plate at a rate of 10 -8 to 10 -7 mol/sec·cm 2 , and the shutter in front of the coated plate was opened. Thus, the vapor deposition of the above monomers on the ITO-coated plate was carried out. When the thickness of the vapor deposition film on the coated plate reached 50 nm (500 Å) as measured by a quartz resonator film thickness meter, the shutter was again closed. The plate holder was heated to a temperature of 300 °C to thermally treat the vapor deposition film-coated plate for 30 min. This treatment completed the polymerization of the above 5-diphenylamino-isophthaloyl chloride and terephthaloyl dihydrazide and of the above 5-tert-butylisophthaloyl chloride and terephthaloyl dihydrazide. Thus, two mutually different thin polyoxadiazole films were prepared on the ITO of the ITO-coated glass plate. Confirmation of Formation of Two Mutually Different Thin Polyoxadiazole Films The vapor deposition polymerization of 5-diphenylamino-isophthaloyl chloride and terephthaloyl dihydrazide was conducted on an Al plate having a thickness of 0.5 mm to thereby form a polymeric thin film having a thickness of 1 µm. Likewise, the vapor deposition polymerization of 5-tert-butylisophthaloyl chloride and terephthaloyl dihydrazide was conducted on an Al plate having a thickness of 0.5 mm to thereby form a polymeric thin film having a thickness of 1 µm. FT-IR spectra of the specimen were taken by the reflection method. The spectra showed the disappearances of absorption at 3212 cm -1 characteristic of a hydrazide group (N-H stretching vibration) and of the C=O stretching vibration of a carbonyl group at 1666 cm -1 to thereby ensure the formation of oxadiazole rings. The FT-IR spectra of the thin films obtained by the above vapor deposition polymerization agreed with that of a thin film obtained by the solution polymerization technique. Preparation of Electroluminescent Device and Confirmation of Electroluminescence A counter electrode of an MgAg alloy having a weight ratio of Mg to Ag of 10:1 was formed by codeposition on the two mutually different thin polyoxadiazole films superimposed on the ITO of the ITO-coated glass plate to thereby prepare an electroluminescent device. Thus, a thin-film electroluminescent device 10 as illustrated in Fig. 3 was obtained. Using the ITO electrode as a positive electrode 2 and the MgAg electrode as a negative electrode 1, a direct current voltage of 10 V was applied between the electrodes 1 and 2. The electroluminescent device produced emits blue-green electroluminescence (EL). Example 8 The silane monomer represented by the formula: i.e., bis (p-chlorocarbonylphenyl) dimethylsilane, was placed in a first vapor source in a vapor deposition polymerization apparatus. Further, commercially available 4,4'-biphenyldicarbonyl dichloride and terephthaloyl dihydrazide (produced by Wako Pure Chemical Industries, Ltd.) were placed in a second and a third vapor source, respectively in the vapor deposition polymerization apparatus. A glass plate provided with an electrode of ITO (indium tin oxide), having a thickness of 100 nm (1000 Å) (manufactured by Hoya Corp.) was subjected to ultrasonic cleanings successively using acetone, superpurified water, plate cleaner (Semicoclean, grade EL, produced by Furuuchi Kagaku), superpurified water and isopropyl alcohol (IPA). From isopropyl alcohol vapor, the plate was taken out and dried. The resultant electroderovided glass plate was mounted on a temperature-controllable plate holder disposed in a vapor deposition polymerization apparatus. The inside of the vapor deposition polymerization apparatus was evacuated to a total pressure of 1 x 10 -3 Pa or below by an oil diffusion pump. In the beginning, a shutter disposed in front of the coated plate for isolating the coated plate from the vapor sources was closed, and, while keeping the shutter closed, the vapor sources were heated by the resistance heating method. The respective temperatures of the vapor sources were set while monitoring the rates of evaporation of the monomers from the respective vapor sources. Shutters for the vapor sources were opened, and the vapor deposition was conducted under the control such that the silane monomer and the terephthaloyl dihydrazide were simultaneously evaporated at at 10 -8 to 10 -7 mol/sec·cm 2 and deposited on the electrode-provided glass plate. When the thickness of the vapor deposition film on the plate reached 15 nm (150 Å) as measured by a quartz resonator film thickness meter, the shutter for the silane monomer was again closed. Concurrently, the shutter for the 4,4'-biphenyldicarbonyl dichloride vapor source was opened to start evaporation thereof. At this time, the rates of evaporation of terephthaloyl dihydrazide and 4,4'-biphenyldicarbonyl dichloride were set at 10 -8 - 10 -7 mol/sec·cm 2 . When the thickness of the vapor deposition film on the above vapor deposition film reached 60 nm (600 Å), all the shutters for the vapor sources were closed. The plate holder was heated to a temperature of 300 °C to thermally treat the thus formed three vapor deposition films for 30 min. This treatment completed the polymerization reactions. (1) Confirmation of Formation of Poly(4,4'-biphenylene-2,5-oxadiazolylene) by Polymerization of 4,4'-biphenyldicarbonyl Dichloride and Terephthaloyl Dihydrazide The same vapor deposition polymerization of 4,4'-biphenyldicarbonyl dichloride and terephthaloyl dihydrazide as above was conducted on an Al plate having a thickness of 0.5 mm to thereby form a vapor deposition polymerization film (specimen) having a thickness of 1 µm. An FT-IR spectrum of the specimen was taken by the reflection method. The spectrum showed the disappearances of absorption at 3212 cm -1 characteristic of a hydrazide group (N-H stretching vibration) and of the C=O stretching vibration of a carbonyl group at 1666 cm -1 and further showed the appearances of absorptions at 2857 cm -1 ascribed to aromatic C-H, 1428 - 1560 cm -1 ascribed to >C=N- or >C=C< and 1000 - 1025 and 952 - 975 cm -1 ascribed to =C-O-C= to thereby ensure the formation of an oxadiazole ring. Thus, the formation of poly(4.4'-biphenylene-2,5-oxadiazolylene) by the polymerization of 4,4'-biphenyldicarbonyl dichloride and terephthaloyl dihydrazide was confirmed. (2) Confirmation of Formation of Polymer by Polymerization of the Above Silane Monomer. i.e.. Di(p-chlorocarbonylphenyl)dimethylsilane and Terephthaloyl Dihydrazide A specimen was prepared in the same manner as in item (1) above. An FT-IR spectrum of the specimen was taken by the reflection method. The spectrum showed the disappearances of the N-H stretching vibration at 3212 cm -1 characteristic of a hydrazide group and of the C-O stretching vibration of a carbonyl group at 1795 cm -1 and further showed the appearances of absorptions at 2857 cm -1 ascribed to aromatic C-H, 1428 - 1560 cm -1 ascribed to >C-N- or >C=C< and 1000 - 1025 and 952 - 975 cm -1 ascribed to =C-O-C= to thereby ensure the formation of an oxadiazole ring. Thus, the formation of a polymer by the polymerization of di(p-chlorocarbonylphenyl)dimethylsilane and terephthaloyl dihydrazide was confirmed. In the evaporation of the monomers, the rates of evaporation of the monomers were regulated so as to form a stoichiometric thin film, and especially controlled so as for the total molar amount of the silane monomer and the terephthaloyl dichloride to be equal to the molar amount of the terephthaloyl dihydrazide. The surface of each of the thus formed vapor deposition films was observed by a scanning electron microscope (SEM). The observation showed that the films had excellent flatness and were thin films free of pin holes. A counter electrode of an MgAg alloy having a weight ratio of Mg to Ag of 10:1 was formed by codeposition on the electroluminescent layer superimposed on the electrode-provided glass plate to thereby obtain an electroluminescent device. This electroluminescent device (sample) was electrically connected using the ITO electrode as a positive electrode and the MgAg electrode as a negative electrode, and a direct current voltage of 8 V was applied. The electroluminescent layer produced electroluminescence (EL) having a peak wavelength at 450 nm. This electroluminescent device was continuously driven at room temperature for 50 hr, but no peeling occurred at any of the interfaces of the ITO electrode, the layer comprising the silicon-atom containing polymeric compound and the electroluminescent layer. In the above electroluminescent device, the thickness of the layer comprising the silicon-atom containing polymeric compound (layer with content gradient) sandwiched between the ITO electrode and the electroluminescent layer composed of poly(1,4-phenyl-ene-2,5-oxadiazolylene) was 15 nm (150 Å), the thickness of the electroluminescent device layer composed of poly(1,4-phenyl-ene-2,5-oxadiazolylene) was 60 nm (600 Å) and the thickness of the MgAg electrode was 100 nm (1000 Å). Example 9 Production of Thin-film Electroluminescent device Comprising Electroluminescent Device Layer in which Ratio in Content of Silicon-containing Polymer to Polycondensed Polymer Has Gradient The silane monomer of formula (S1) and the following silane monomer of formula: i.e., bis (p-chlorocarbonylpyridyl)silane, terephthaloyl dichloride and terephthaloyl dihydrazide were put in separate vapor sources. The electrode-provided glass plate having been cleaned in the same manner as in Example 1 was mounted on a temperature-controllable plate holder disposed in a vapor deposition polymerization apparatus. The inside of the vapor deposition polymerization apparatus was evacuated to a total pressure of 1 x 10 -3 Pa or below by an oil diffusion pump. In the beginning, a shutter disposed in front of the plate for isolating the plate from the vapor sources was closed, and, while keeping the shutter closed, the vapor sources were heated by the resistance heating method. The respective temperatures of the vapor sources were set while monitoring the rates of evaporation of the monomers from the respective vapor sources. Shutters for the vapor sources were opened, and the vapor deposition was conducted under the control such that the silane monomer of formula (S1) and the terephthaloyl dihydrazide were simultaneously evaporated at a rate of 10 -8 to 10 -7 mole/sec·cm 2 and deposited on the electrode-provided glass plate. When the thickness of the vapor deposition film on the plate reached 5 nm (50 Å) as measured by a quartz resonator film thickness meter, all of the shutters were again closed. Vapor deposition conditions were regulated so as for the total molar amount of the silane monomer of formula (S1) and terephthaloyl dichloride to be equal to the molar amount of terephthaloyl dihydrazide. Then, all of the shutters for three of the vapor sources were opened. The mole ratio of the silane monomer of formula (S1) and terephthaloyl dichloride was controlled to be changed continuously by gradual decreasing the evaporated amount of the silane monomer and increasing the evaporated amount of terephthaloyl dichloride while maintaining the total moles of the silane monomer and terephthaloyl dichloride equal to the moles of terephthaloyl dihydrazide. When a vapor deposition film having a thickness of 20 nm (200 Å) was formed on the above vapor deposition film, all of the shutters were again closed. The rates of evaporation of terephthaloyl dichloride and terephthaloyl dihydrazide were regulated, and the shutters therefor were opened. When a vapor deposition film having a thickness of 15 nm (150 Å) was formed, the shutters for the vapor sources were closed. Then, the total moles of the silane monomer represented by the formula (S2) and terephthaloyl dichloride were controlled to become equal to the moles of terephthaloyl dihydrazide, followed by opening the shutters for three of these vapor sources. The mole ratio of the silane monomer of formula (S2) was continuously changed by increasing the mole ratio of the silane monomer to become only the silane monomer at the end while maintaining the total moles of the silane monomer and terephthaloyl dichloride equal to the moles of terephthaloyl dihydrazide. The plate holder was heated to a temperature of 300 °C to thermally treat the thus formed three vapor deposition films for 30 min. This treatment completed the polymerization reactions. The formation of the polymers was confirmed in the same manner as in Example 8. An MgAg electrode for injecting electrons was formed by codeposition on the electroluminescent layer superimposed on the electrode-provided glass plate to thereby obtain an electroluminescent device. This electroluminescent device (sample) was electrically connected using the ITO electrode as a positive electrode and the MgAg electrode as a negative electrode, and a direct current voltage of 10 V was applied. The electroluminescent layer produced electroluminescence (EL) having a peak wavelength at 450 nm. This electroluminescent device was continuously driven at room temperature for 40 hr, but no peeling occurred at any of the interfaces of the ITO electrode, the layer comprising the silicon-atom containing polymeric compound, the electroluminescent layer, the layer comprising the silicon-atom containing polymeric compound and the electrodes.

Claims: A thin-film electroluminescent device comprising an electroluminescent layer interposed between electrodes, at least one thereof being transparent, wherein the electroluminescent layer comprises a polyoxadiazole derivative, formed by the vapor deposition polymerization process, represented by the formula: wherein each of X 0 and Y 0 independently represents a divalent organic group which may comprise a silicon atom, and n is an integer of 2 or greater. A thin-film electroluminescent device comprising an electroluminescent layer interposed between electrodes, at least one thereof being transparent, and a charge injecting/transporting layer interposed between at least one of the electrodes and the electroluminescent layer, wherein at least one of the electroluminescent layer and the charge injecting/transporting layer comprises a polyoxadiazole derivative formed by the vapor deposition polymerization process, represented by the formula (I) as defined in claim 1. A device according to claim 2, wherein the electroluminescent layer comprises a polyoxadiazole derivative formed by the vacuum deposition polymerization process, represented by the formula I as defined in claim 1, and wherein the charge injecting/transporting layer comprises polyoxadiazole derivatives formed by the vapor deposition polymerization process, represented by the formula: wherein each of X 0 and Y 0 independently represents a divalent organic group which may comprise a silicon atom, and n is an integer of 2 or greater, provided that at least one of X 0 and Y 0 of formula (I) is different from both of X 0 and Y 0 of formula (II). A device according to any one of the preceding claims which emits a fluorescence having a wavelength of 400 to 600 nm. A device according to any one of the preceding claims wherein the electroluminescent layer comprises a polyoxadiazole derivative of formula (I) and a fluorescent dye or pigment in an amount of 0.01 to 10 % by mole per repeating unit of the polyoxadiazole derivative. A device according to any one of claims 2 to 5 wherein the charge injecting/transporting layer comprises a polyoxadiazole derivative of formula (I), and a charge injecting/transporting additive in an amount of 0.01 to 80% by mole per repeating unit of the polyoxadiazole. A device according to claim 6 wherein the charge injecting/transporting additive is a diphenoquinone or fluorenone derivative. A device according to claim 6 wherein the charge injecting/transporting additive is a triphenylamine derivative. A device according to claim 8 wherein the triphenylamine derivative is 4,4',4"-tris(N,N-diphenylamino)triphenylamine or 4,4',4"-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine. A device according to any one of claims 2 to 9 wherein the charge injecting/transporting layer comprises a silicon-atom containing polyoxadiazole derivative represented by the formula: wherein each of X 1 and Y 1 independently represents a divalent organic group, at least one of X 1 and Y 1 being a divalent organic group having a silicon atom, and n is an integer of 2 or greater. A device according to claim 10 wherein the charge injecting/transporting layer further comprises a polyoxadiazole derivative containing no silicon atom represented by the formula: wherein each of X 2 and Y 2 independently represents a divalent organic group containing no silicon atom, and n is an integer of 2 or greater, and wherein the content of the silicon-atom containing polyoxadiazole derivative in the charge injecting/transporting layer decreases from the electrode toward the electroluminescent layer, while the content of the polyoxadiazole derivative containing no silicon atom increases from the electrode towards the electroluminescent layer. A process for preparing a device as defined in any one of the preceding claims comprising: (i) forming an electrode on a plate; (ii) optionally forming the first charge injecting/transporting layer on the electrode by the vapor deposition polymerization process; (iii) forming the electroluminescent layer on either the electrode or the first charge injecting/transporting layer by the vapor deposition polymerization process; (iv) optionally forming the second charge injecting/transporting layer capable of injection and transport of charges opposite to those injected in and transported by the first charge injecting/transporting layer on the electroluminescent layer by the vapor deposition polymerization process; and (v) forming a counter electrode on either the electroluminescent layer or the second charge injection/transporting layer.

Inventors: FUJIYAMA TAKAHIRO, MURATA HIDEYUKI, SAITO SHOGO, TSUITSUI TETSUO, YAMANAKA TOORU, FUJIYAMA, TAKAHIRO, MURATA, HIDEYUKI, SAITO, SHOGO, TSUITSUI, TETSUO, YAMANAKA, TOORU, FUJIYAMA, TAKAHIRO, C/O MITSUI PETR.IND.LTD., MURATA, HIDEYUKI, C/O MITSUI PETR.IND.LTD., YAMANAKA, TOORU, C/O MITSUI PETR.IND.LTD.
Applicants: MITSUI PETROCHEMICAL IND, MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1994
Main classification: H05B33
IPC codes: H05B33, H01L51, C09K11